1. Field of the Invention
The present invention relates to a differential that transmits a driving force of an energy conversion machine, such as an engine or a motor, to a wheel driving shaft.
2. Description of the Related Art
Conventionally, a three-wheeled or four-wheeled vehicle uses a driving-force transmission apparatus in which the driving force of an energy conversion machine, such as an engine or a motor, is controlled while being supplied to a transmission, this controlled driving force is then supplied from an output shaft of the transmission to a differential through a propeller shaft when necessary, and the driving force of the energy conversion machine is transmitted through the differential to a wheel driving shaft. For example, in an FF type driving-force transmission apparatus, the differential of the driving-force transmission apparatus is rotatably disposed in a mission case of a transmission used as a housing through a bearing.
A 4-pinion type differential having four pinions has been proposed as the differential of the driving-force transmission apparatus, for example, because a high load imposed on pinions, which results from the fact that the energy conversion machine has been designed to output a higher driving force in recent years, can be dispersed to reduce a load assigned to one of the pinions.
Additionally, in recent years, a differential having a closed differential case has been proposed, for example, because it is possible to easily reduce a noise caused by an outward leak of a sound emitting from a sliding portion formed in the differential case, such as an operational noise of a differential gear made up of a pinion and a side gear.
Therefore, a proposal has been made for a differential that is rotatably held on a pinion shaft formed by cross-shaped four pinions in a closed differential case.
The following two kinds of differentials can be mentioned as the conventional 4-pinion type differential that uses a closed differential case.
A first conventional differential is structured to be rotatably held on a pinion shaft integrally formed with cross-shaped four pinions in a closed differential case.
A second conventional differential is structured to rotatably support four pinions 105 by arranging a long pinion shaft 102 and two short pinion shafts 103 and 104 like a cross in a case 101 of a differential 100, as shown in FIG. 1.
In the thus structured differential 100, a case 101 has four through-holes 106 in four directions, i.e., in upward, downward, leftward, and rightward directions centering on its rotational axis center RL, as shown in FIG. 1. These through-holes 106 are equi-angularly spaced from each other at angles of 90 degrees centering on the rotational axis center RL of the case 101 so that the distance between the adjoining ones becomes equal. The pinion shafts 102, 103, and 104 are fitted to the through-holes 106, respectively, from the outside of the case 101. These pinion shafts 102, 103, and 104 are fixed to the case 101 by a fixing pin 107 fitted and fixed to the case 101.
In greater detail, the long pinion shaft 102 has its both ends fitted to the through-hole 106, and, at a part forming the through-hole 106 on the side of the upper end of the case 101 shown at the upper part of FIG. 1, the fixing pin 107 fitted to the case 101 can prevent the long pinion shaft 102 from rotating upon the axis center. Furthermore, the short pinion shaft 103 shown on the left of FIG. 1 has its left end fitted to the through-hole 106 and has its right end positioned on the side of the rotational axis center RL of the case 101. At a part forming the through-hole 106 on the side of the left end of the case of FIG. 1, the fixing pin 107 fitted to the case 101 can prevent the short pinion shaft 103 from rotating upon the axis center. Furthermore, the short pinion shaft 104 shown on the right of FIG. 1 has its right end fitted to the through-hole 106 and has its left end positioned on the side of the rotational axis center RL of the case 101. At a part forming the through-hole 106 of the right end part of the case of FIG. 1, the fixing pin 107 fitted to the case 101 can prevent the short pinion shaft 104 from rotating upon the axis center.
A supporting ring 108 is provided in the case 101. The long pinion shaft 102 is disposed to radially pass through the center of the supporting ring 108. The supporting ring 108 supports the ends of the two short pinion shafts 103 and 104 situated on the side of the rotational axis center RL of the case 101.
A cross-shaped pinion shaft unit 109 is made up of the three pinion shafts 102, 103, and 104, the three fixing pins 107, and the supporting ring 108. This pinion shaft unit 109 is shown in FIG. 2.
For example, in an FF type driving-force transmission apparatus, the differential of the driving-force transmission apparatus is rotatably disposed through a bearing in the mission case of the transmission used as a housing. When a closed differential case is used, a lubrication structure is formed so that a lubricant supplied to a sliding portion formed in the transmission can be supplied to a sliding portion formed in the differential case through a small gap formed between the surface of the differential case and the surface of the wheel driving shaft facing the surface of the differential case by forced lubrication that uses an oil pump driven by a driving force of the driving-force transmission apparatus or by splash lubrication that uses splashes of a lubricant.
However, in the conventional differential, although a lubricant can be supplied into the closed differential case, the quantity of the lubricant supplied thereinto is small, and it is insufficient to prevent all of the heat and wear of a sliding portion of the differential. Additionally, since the quantity of the lubricant supplied into the differential case is small, the lubricant will deteriorate in a relatively short time. As a result, disadvantageously, a stable function cannot be maintained for a long time.
In order to settle this problem, a proposal has been made for a differential that has a lubricant supply means provided with a lubricant supply hole through which, when the differential case is rotated, a lubricant is guided from the outside of the differential case into the differential case in the outer peripheral surface of a closed differential case and that can forcedly lubricate the interior of the differential case with a part of the lubricant used for lubrication of a sliding portion of the transmission by the lubricant supply means when the differential case is rotated by forced lubrication that uses an oil pump driven by a driving force of the driving-force transmission apparatus or by splash lubrication that uses splashes of a lubricant.
However, an outstanding problem resides in the fact that the conventional differential cannot satisfy the recent demand for high performance and cost reductions.
For example, current vehicles are required to be superior in performance, and, accordingly, a differential has been designed to exhibit high performance. In order to achieve a differential having such high performance, an improvement in durability and a reduction in abnormal noises have been demanded by optimizing the engagement between differential gears.
The engagement between differential gears can be optimized by lessening gaps generated by a backlash at the engagement portion between a side gear and a pinion as much as possible and then evening out the gaps.
However, disadvantageously, in the conventional differential, it is difficult to evening out gaps between surfaces facing each other at the engagement portion between the side gear and the pinion, and a gap for a backlash must be enlarged.
Additionally, in the conventional differential, it is difficult to evening out a gap between mutually opposite surfaces of the side gear to which a wheel driving shaft is attached and the differential case. In greater detail, a gap is formed between a back surface of the side gear and an inner surface of the differential case that faces the back surface thereof, and therefore gaps in the axial direction of a pair of wheel driving shafts connected to the side gear have differed. As a result, disadvantageously, the movement amount in the axial direction of the pair of wheel driving shafts become different, and rickety movements of wheels and tires do not become even for the right and left, consequently lowering its commercial value.
These problems prominently arise when the pinion shaft is positioned and fixed to the differential case with high accuracy.
The aforementioned gaps can, of course, be theoretically evened out. However, to do so, constituent elements of the differential must be constructed with even higher accuracy, and many man-hours are required for their assembly. However, this cannot satisfy the demand for cost reductions in recent years, and is unrealistic.
Therefore, a demand has been made for a differential capable of easily and reliably evening out gaps between mutually opposite surfaces at the engagement portion among a side gear and three or more pinions and capable of achieving an improvement in durability and a reduction in abnormal noises by optimizing the engagement of differential gears.
Additionally, in recent years, various devices of a vehicle have been required to be reduced in cost. A differential is mentioned as one of the devices to be reduced in cost. However, the first conventional differential of the 4-pinion type that uses a closed differential case has a problem in the fact that many processing steps are necessitated to form a cross-shaped pinion shaft while spending many man-hours, thus making it impossible to satisfy the demand for cost reductions.
Likewise, the second conventional differential 100 has a problem in the fact that there are a great many components and a significant number of assembly steps, thus making it impossible to satisfy the demand for cost reductions.
In the lubrication structure in which forced lubrication of the interior of the closed differential case is performed by the lubricant supply means when the differential case used in the conventional differential is rotated, the sliding portion can be excellently lubricated by supplying a lubricant to the interior of the differential case by use of the lubricant supply means when the differential case is rotated, i.e., when a vehicle is traveling. However, a lubricant is not supplied to the interior of the differential case when the differential case stops rotating, i.e., when the vehicle stops traveling. Therefore, a problem resides in the fact that, when the vehicle is started, the quantity of a lubricant to be supplied is small, and the possibility that excellent lubrication cannot be obtained arises.
In other words, in general, when the vehicle is started, a lubricant is supplied to the sliding portion formed in the mission case, for example, through an oil pump rotated and driven by a driving force of the driving-force transmission apparatus, and a part of the lubricant that has passed through the bearing is supplied to the interior of the differential case through the lubricant supply hole of the lubricant supply means. However, in effect, there is a time-lag until the lubricant reaches the interior of the differential case, and, since a rotational speed is low when started, the quantity of a lubricant discharged from the oil pump is small, and therefore the quantity of a lubricant to be supplied to the interior of the differential case is small. Additionally, the quantity of a lubricant reserved in the differential case when the vehicle is traveling depends on a traveling state of the vehicle and other factors.
Therefore, when the vehicle is started, only the lubricant reserved in the differential case when the vehicle was traveling immediately before its start contributes to the lubrication of the differential. When the quantity of the lubricant reserved in the differential case is decreasing, it is insufficient to prevent all of the heat and wear of the sliding portion of the differential, and the sliding portion of the differential is greatly worn down, and the lubricant deteriorates. Especially, when the vehicle curves or turns before a lubricant is supplied to the interior of the differential case, wear-out of the sliding portion and deterioration of the lubricant remarkably appear. If the operation time of the differential gear becomes long at this time, there is a fear that the differential gear will be burned and damaged.
Additionally, the lubrication structure of the conventional differential necessitates an oil pump used to raise the lubricant reserved in the mission case and an oil passage used to guide the lubricant raised by the oil pump to the bearing or a device used to splash the lubricant reserved in the mission case, thus causing the problem of hindering size reductions and cost reductions.